1. Field of the Invention
This invention relates to electrical device comprising PTC conductive polymers.
2. Introduction to the Invention
Conductive polymer compositions exhibiting PTC behavior, and electrical devices comprising them, are well comprising PTC conductive, for example, to U.S. Pat. Nos. 2,952,761; 2,978,665; 3,243,753; 3,351,882; 3,571,777; 3,757,086; 3,793,716; 3,823,217; 3,858,144; 3,861,029; 3,950,604; 4,017,715; 4,072,848; 4,085,286; 4,117,312; 4,177,376; 4,177,446; 4,188,276; 4,237,441; 4,242,573; 4,246,468; 4,250,400; 4,252,692, 4,255,698, 4,271,350, 4,272,471, 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,314,231, 4,315,237, 4,317,027, 4,318,881, 4,327,351, 4,330,704, 4,334,351, 4,352,083, 4,388,607, 4,398,084, 4,435,639, 4,429,216, and 4,442,139; J. Applied Polymer Science 19, 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18, 649-653 (1978), Narkis et al; and commonly assigned U.S. Ser. Nos. 601,424 now abandoned, published as German OLS No. 1,634,999; 732,792 (Van Konynenburg et al), now abandoned, published as German OLS No. 2,746,602; 798,154 (Horsma et al), now abandoned, published as German OLS No. 2,821,799; 134,354 (Lutz); 141,984 (Gotcher et al), published as European Application No. 38,718; 141,988 (Fouts et al), published as European Application No. 38,718, 141,989 (Evans), published as European Application No. 38,713, 141,991 (Fouts et al), published as European Application No. 38,714, 150,909 (Sopory), published as UK Application No. 2,076,106A, 184,647 (Lutz), 250,491 (Jacobs et al) now abandoned in favor of Continuation Application Ser. No. 656,046 and 254,352 (Taylor), published as European Application No. 63,440, 272,854 and 403,203 (Stewart et al), published as European Patent Application No. 67,679, 274,010 (Walty et al), 300,709 and 423,589 (van Konynenburg et al), published as European Application No. 74,281, 349,505 (McTavish et al), published as European Application No. 87,884, 369,309 (Midgley et al), 380,400 (Kamath), published as European Application No. 96,492, 474,390 (Leary), 483,633 (Wasley), 485,572 (Nayak et al), 493,445 (Chazan et al), 493,390 (Leary et al), 509,897 (Masia et al), 524,482 (Tomlinson et al), 534,913 (McKinley), 535,449 (Cheng et al) 552,649 (Jensen et al), 573,099 (Batliwalla et al), 904,736, published as UK Pat. Nos. 1,470,502 and 1,470,503, 628,945 (Carlomagno), and 650,918 and 650,920 (Batliwalla et al) (MP0959, 961 and 962). The disclosure of each of the patents, publications and applications referred to above is incorporated herein by reference.
Particularly useful devices comprising PTC conductive polymers are self-regulating heaters and circuit protection devices. Self-regulating heaters are hot and have relatively high resistance under normal operating conditions. Circuit protection devices are relatively cold and have a relatively low resistance under normal operating conditions, but are "tripped", i.e., converted into a high resistance state, when a fault condition, e.g., excessive current or temperature, occurs. When the device is tripped by excessive current, the current passing through the PTC element causes it to self-heat to an elevated temperature at which it is in a high resistance state. Circuit protection devices and PTC conductive polymer compositions for use in them, are described for example in U.S. Pat. Nos. 4,237,411, 4,238,812, 4,255,698, 4,315,237, 4,317,017, 4,329,726, 4,352,083, 4,413,301, 4,450,496, 4,475,138, 4,481,498, and 4,562,313; in U.S. application Ser. No. 254,352 which is now U.S. Pat. No. 4,426,633; and in copending, commonly assigned U.S. application Ser. Nos. 141,989 (MP0715), and 754,807 (MP0906). Other applications which are related to this application are the copending, commonly assigned applications filed contemporaneously with this application by Deep et al, Ser. No. 711,909 (MP1022), by Carlomagno, Ser. No. 711,790 (MP0991), now U.S. Pat. No. 4,685,025, by Ratell, Ser. No. 711,907 (MP1021), now U.S. Pat. No. 4,647,894, and by Ratell, Ser. No. 711,908 (MP1016), now U.S. Pat. No. 4,647,896. The disclosure of each of these patents and prior filed pending applications is incorporated herein be reference.
In many devices, and especially in circuit protection devices, it is desirable or necessary for the PTC conductive polymer to be cross-linked, preferably by means of radiation. The effect of the cross-linking depends on, among other things, the polymer and the conditions during the cross-linking step, in particular the extent of the cross-linking, as discussed for example in copending, commonly assigned U.S. application Ser. No. 468,768, the disclosure of which is incorporated herein by reference. When a conductive polymer element is irradiated, the radiation dose absorbed by a particular part of the element in a given time depends upon its distance from the surface of the element exposed to the source, and the intensity, energy and type of the radiation. For a relatively thin element and a highly penetrating source (e.g. a Cobalt 60 source), the variation of dose with thickness is negligible. However, when using an electron beam, the variation in dose with thickness can be substantial; this variation can be offset by exposing the element to radiation from different directions, e.g. by traversing the element past the source twice, irradiating it first on one side and then on the other. Depending upon the energy of the beam and the thickness of the element (which can of course vary, depending upon its shape), the radiation dose can be higher at the surfaces exposed to radiation than at the middle, or substantially uniform across the thickness of the element, or higher at the middle than at the surfaces exposed to radiation. In addition, the radiation dose near the surface exposed to the radiation can be less than expected because of surface scattering, and the radiation dose in the vicinity of the electrodes is affected by the shielding effect and the scattering effect of the electrodes.